Angiotensin I converting enzyme (ACE) is an exopeptidase related to the blood pressure control system known as reninangiotensin system. This enzyme converts inactive decapeptide angiotensin I by cleaving carboxylic terminal dipeptide to give active octapeptide angiotensin II, the most potent naturally occurring pressor substance known. The enzyme also inactivates the potent vassodepressor peptide bradikinin. Therefore ACE plays physiologically very important roles in maintenance of the blood pressure system from the elevating side. Much work has been done to find out effective ACE inhibitors in the hope to reduce the blood pressure of essential hypertensive patients of reninangiotensin dependent types. The most effective inhibitor ever developed is SQ 14,225, an artificially synthesized dipeptide known as Captopril. Today Captopril is effectively dosed orally in common clinical therapy for various kinds of essential hypertensions. Yet this compound has only about 10 years history and some side effects are quoted in some cases. Therefore good hypotensive method with mild but sure effect which could be used lifelong without any fear of side effects are wanted.